The cellular homolog (c-fos) of the retroviral transforming gene, v-fos (occurring in FBJ-murine sarcoma virus) is expressed at high levels in a tissue- and stage-specific fashion during the development of murine embryos and occurs also in human fetal development. Extraembryonic tissues express the highest levels (placenta, amnion, visceral yolk sac, etc.), but fetal liver, bone marrow, and skin also express c-fos constitutively at lower levels. These examples and others in the literature demonstrate that c-fos expression can be activated by a variety of stimuli including growth factors, vitamin derivatives, and tumor promoters. In a variety of cell types, c-fos mRNA and protein expression are tightly regulated. The overall aim of this proposal is to study the mechanisms of control and the functions of c-fos with particular emphasis on its role in development. The following approaches will be used. 1) The function of fos will be directly tested by the introduction of "anti-sense" and "sense" fos recombinant DNA into embryonic cells of several types. Transcription of these gene sequences will prevent or enhance (respectively) c-fos expression. 2) c-Fos transcripts will be detected in individual cells and tissues by in situ hybridization of DNA probes to tissue sections and fixed cultured cells. 3) The possibility that the fos gene is activated by DNA demethylation will be investigated. 4) Using antibodies, the levels of fos protein in tissues and the basis for its post-translational modifications will be studied. The range of our present antibodies will be extended in order to follow the dynamics of fos protein regulation and to purity it from different sources. Viral oncogenes are intimately associated with cellular transformation and it is generally thought that the cellular proto-oncogenes also have growth-related functions. The presence of high levels of fos expression in embryonic tissues supports this idea. The studies proposed here take advantage of this to define the rules for its regulation and thereby help to point to a role for c-fos in normal or abnormal growth or differentiation.